OBJECTIVES: To predict and explain the pattern of cruciate-ligament loading during squatting exercises; to determine the effect of hamstrings co-contraction on anterior cruciate ligament (ACL) load during squatting; and to determine the effect of the weightbearing force on ACL load during squatting. DESIGN: Mathematical modeling of the human musculoskeletal system. BACKGROUND: Squatting is a commonly prescribed exercise for strengthening the muscles of the thigh following ACL reconstruction. Although the forces induced in the ACL are purported to be low, no experimental data are available to corroborate this claim. The reason is that measurements of knee-ligament forces are difficult to obtain in vivo. METHODS: The human body was modeled as a four-segment, six-degrees of freedom, planar linkage. The hip, ankle and toes were each modeled as a hinge joint. The relative displacements of the femur, tibia and patella were calculated using a three-degrees of freedom, sagittal-plane model of the knee. Eleven elastic were used to describe the geometric and mechanical properties of the knee ligaments. The model was actuated by 22 musculotendinous units. Optimization theory was used to calculate the forces developed in the muscles and the forces transmitted to the knee ligaments during squatting. RESULTS: The model ACL was loaded from full extension to 10 degrees of knee flexion during squatting; the model PCL was loaded at knee-flexion angles greater than 10 degrees. The pattern of cruciate-ligament loading is determined by the shapes of the articulating surfaces of the bones and by the changing orientation of the hamstrings muscles at the knee. Hamstrings co-contraction is the major determinant of ACL loading during squatting exercises; the weightbearing force has a relatively small effect on the force induced in the ACL. CONCLUSION: The calculations support the contention that squatting is a relatively safe exercise for strengthening the muscles of the thigh following reconstruction of the ACL. RELEVANCE: Knowledge of the forces borne by the knee ligaments is important for designing exercise regimens subsequent to ligament injury and repair. The quadriceps and hamstrings muscles may be strengthened without loading a newly reconstructed ACL by performing squats with the knee flexed to 10 degrees and greater.